1. Field of the Invention
The present invention relates to a circularly polarized patch antenna, and in particular to a circularly polarized patch antenna with single supply point.
2. Discussion of the Related Art
As is known, patch antennas are widely employed in applications that require use of antennas, characterized by small overall dimensions, planar geometry, and constructional simplicity.
In particular, the patch antennas widely available on the market comprise a ground surface and a metal lamina arranged parallel above the ground surface with an interposed dielectric layer; the thickness of the metal lamina is negligible as compared to the thickness of the dielectric layer so that the metal lamina can be generally equated to a two-dimensional object, identifying a plane. A protective casing, typically of plastics, may be arranged around the ground surface and the metal lamina so as to guarantee a protection from atmospheric agents.
The metal lamina represents the radiant component of the patch antenna and is generally supplied by a supply line, that carries signals directed to the metal lamina or coming from the metal lamina. Generally, the supply line comprises a stripline connected to the metal lamina, or a metal via arranged in the dielectric layer orthogonal to the ground surface and to the metal lamina. In the case of a metal via, a connector is generally arranged at one end of the metal via, while the other end is connected to the metal lamina, thereby enabling ohmic contact between the metal lamina and a possible external connection cable coupled to the connector. Typically, the point of contact between the metal via and the metal lamina is chosen so that the impedance represented by the metal lamina and seen by the metal via is approximately equal to 50Ω.
Given an operating wavelength, a patch antenna having a particularly simple geometry comprises a metal lamina of a rectangular shape, wherein a first side, typically the long side, is slightly shorter than half of the operating wavelength, to take into account any non-ideality of distribution of the field on the edges (the so-called “fringing field”). The supply is provided by a stripline connected to a second side of the metal lamina, orthogonal to the first side. The patch antenna with the geometry described irradiates linearly polarized radiation, with an antenna efficiency that depends, among other factors, upon the impedance seen from the supply line, i.e., from the stripline, towards the metal lamina, in particular upon the impedance adaptation between the stripline and the metal lamina. As is known, the metal lamina is sized at a design frequency, generally equal to the nominal operating frequency of the patch antenna, while the supply line is designed to optimize impedance adaptation between the supply line and the metal lamina. Moving away from the design frequency, the levels of performance of the patch antenna decay rapidly.
According to the type of supply line, commonly known patch antennas can be classified into:
patch antennas with two supply points, an illustrative example whereof is shown in FIG. 1, typically supplied by two stripline 50 traversed by respective supply signals;
patch antennas with a single supply point, an illustrative example whereof is shown in FIG. 2, wherein the supply line comprises a metal via 51 extending in the dielectric layer perpendicular to the ground surface and to the metal lamina; and
patch antennas with a single supply point in the plane defined by the metal lamina, as shown in the illustrative examples of FIGS. 3a-3c, wherein the supply line comprises a stripline 52 parallel to the ground surface and lying in the plane defined by the metal lamina.
As regards the polarization of the radiation emitted by the patch antenna, and bearing in mind the classification corresponding to the type of supply, patch antennas are available having geometries that enable irradiation and reception of circularly polarized radiation, said antennas being also known as circularly polarized patch antennas. These patch antennas find wide application in systems that make use of circularly polarized radiation, such as, for example, systems for satellite communications or else systems for automated payment of roadway tolls.
The circular polarization is obtained by using metal laminas provided with portions without metal arranged in a symmetrical way inside the metal lamina; otherwise, there would be the risk of receiving just a part of the circularly polarized radiation.
Circularly polarized patch antennas are also known, comprising a rectangular metal lamina, the lengths of the sides whereof respect a given ratio. Generally, patch antennas of this type have a single supply point and use a metal via.
Further circularly polarized patch antennas comprise a square metal lamina and two supply points, in addition to supply lines provided with power dividers and phase-shifters.
To avoid the use of two supply lines, albeit maintaining the circular polarization, circularly polarized patch antennas have been introduced comprising a lamina having a square shape chamfered at the vertices. In particular, use is known of a square metal lamina provided with two equal chamfers arranged on two opposite vertices of the square identified by the metal lamina, i.e., arranged in a symmetrical way with respect to a diagonal of the metal lamina, as illustrated in FIGS. 3a and 3b. The presence of the chamfers enables degeneration of the orthogonal modes of the antenna and, consequently, emission/reception of circularly polarized radiation.
As illustrated once again in FIGS. 3a and 3b, circularly polarized patch antennas comprising a lamina having the shape of a square chamfered at the vertices typically have a single supply point in the plane defined by the metal lamina; the supply is provided by a stripline line. Given this particular supply line, the geometry of the metal lamina is usually designed not only on the basis of an optimization of the polarization of the irradiated/received radiation, but also on the basis of the impedance adaptation between the supply stripline line and the metal lamina. As illustrated in FIGS. 3a-3c, very sophisticated geometries of the metal lamina are employed, comprising recesses at the edges of the perimeter of the metal lamina, housing part of the stripline line, so that the stripline line is ohmically connected to the metal lamina not at one of the sides of the lamina, but rather inside the aforementioned perimeter. In greater detail, the point of connection between the metal lamina and the supply line is designed so as to optimize the impedance adaptation between the metal lamina and the stripline supply line. Since stripline lines are generally sized so as to have a characteristic impedance of approximately 50Ω, the point of connection is designed so that the impedance represented by the metal lamina seen by the stripline line from the point of connection is approximately 50Ω. To enable a better adaptation, metal laminas are known that have, in addition to the recess housing the stripline line, further recesses housing metal stubs, as illustrated in FIGS. 3b and 3c. 
Known circularly polarized patch antennas are not free from disadvantages. In particular, circularly polarized patch antennas with two supply points generally require the use of power dividers/adders and of 90-degree phase-shifters, i.e., additional elements that are costly as compared to the patch antenna. Sizing of these additional components is frequently problematical, since both the power dividers/adders and the phase-shifters generally have dimensions comparable with the dimensions of the metal lamina and can cause perturbation in the irradiation diagram of the patch antenna.
Furthermore, circularly polarized patch antennas with a single supply point that use a metal via, albeit not requiring additional components, frequently present higher production costs, since the provision of the metal via inside the dielectric layer can entail greater structural complexity.
As regards circularly polarized patch antennas with a single supply point in the plane defined by the metal lamina, they are difficult to implement at high frequency, since to have impedance adaptation between the stripline line and the metal lamina, and in particular to have a stripline line with a characteristic impedance of 50Ω, it would be necessary to use a metal lamina of the patch antenna having a width comparable with the dimensions of the stripline line.